DC to DC converters provide stable regulated supply voltages for operating processors, ASICS, memory, and other circuitry. Emulated current mode DC/DC converters use emulated inductor current information from an RC network to supplement output voltage feedback for comparison with a reference voltage for closed loop pulse width modulation (PWM) control of high and low side converter switches. FIG. 1 shows an on-time controlled DC to DC converter 10 providing an output voltage Vout to a load 2 using high and low side switches S1 and S2 connected in series with one another between an input voltage Vin and a circuit ground 3, with a switching node SW connecting the two switches together and an output inductor 4 connected between the switching node SW and the converter output. Switching control signals are provided to the high and low side switches S1 and S2 in alternating fashion by a driver circuit 5 according to an output provided by a loop comparator 7. The loop comparator 7 receives a voltage reference VREF and a feedback signal FB, along with a differential emulated current input CSP, CSN from an emulated inductor current circuit 100, and provides closed loop regulation of the output voltage Vout through pulse width modulation control of the switches S1 and S2. The circuit 100 includes a current sensor provided by RS1 and CS1 providing a signal indicated emulated inductor current information, as well as a second RC network including RS2 and CS2 for removing DC inductor current information to reduce regulation effects associated with the inductor DC resistance.
For on-time control of the converter 10, the valley voltage of the positive current sense input CSP is used by the PWM comparator 7, leading to unintended switching due to undershoot in discontinuous conduction mode (DCM) operation, as well as undesirable output offset during continuous conduction mode (CCM) operation. FIGS. 2 and 3 illustrate these problems, where a graph 120 in FIG. 2 shows the differential signal CSP, CSN provided by the emulated inductor current circuit 100 of FIG. 1 in DCM mode 122 and CCM mode 124. As seen in FIG. 2, a switching event during DCM operation 122 leads to the CSP signal undershooting the voltage level of the CSN signal. A graph 130 in FIG. 3 shows the reference voltage signal curve 132 (VREF), the output voltage feedback signal curve 134 (FB) as well as a curve 136 showing the summation (SUM) of the voltage feedback FB and the differential emulated current input CSP, CSN. In addition, graph 140 in FIG. 3 shows a curve 142 illustrating the differential voltage input signal VCSP-VCSN, and graph 150 illustrates a curve 152 showing the inductor current for DCM operation 122 and CCM operation 124. As seen in graph 140 of FIG. 3, the initial switching operations 138 of the high side switch S1 during DCM operation 122 cause an initial undershoot with the curve 142 undershooting by an amount 126 below zero (VCSN>VCSP, also seen at 126 in FIG. 2), leading to a pair of subsequent switching operations shown at 144 in graph 140 and 154 in the inductor current graph 150 due to the closed loop operation of the loop comparator 7. This undesirable ringing operation leads to switching noise in the converter output voltage Vout, and the undershoot 126 causes a multi-pulse issue at the CCM/DCM boundary when the output capacitance across the load 2 is large and the output voltage rise per single switching becomes smaller than the undershoot. In addition, the operation of the emulated inductor current circuit 100 in the converter 10 of FIG. 1 leads to the lowermost or “valley” voltage of the CSP having an offset 128 corresponding to the half of ripple voltage during CCM operation 124, as seen in FIGS. 2 and 3. In this regard, the offset is almost zero in DCM operation 122, and thus switching from DCM operation 122 to CCM operation 124 causes an increase in the converter regulation point. Thus, while removing the DC component from the emulated inductor current information advantageously avoids regulation based on the inductor DC resistance, improved DC to DC converters and PWM controller inductor current emulation circuitry are desirable to mitigate or avoid different regulation points in DCM and CCM operation as well as unintended switching operations caused by undershoot in DCM operation.